The present invention relates in general to transmissions and more particularly to a continuously variable speed transmission that employs a power split.
Transmissions have been widely used to regulate speed and torque while keeping engines or prime movers running at desired speeds and torque usually within a narrow range. Speed and torque regulations can be achieved either by largely mechanical means or, alternatively, by electrical motor-generators and associated controls.
There are basically two major types of mechanical transmissions in use today: stepwise transmissions, such as conventional manual and automatic transmissions, and stepless transmissions or CVTs (continuous variable transmissions), such as belt CVTs and toroidal CVTs.
Stepwise transmissions, using multiple gear sets and clutching devices, are quite popular. However, they have the disadvantage of torque interruption while changing gears. Optimization of engine performance for improved fuel economy or pollution reduction often requires an increasing number of gear sets and complex clutching mechanisms. This adds to the cost, complexity and parasitic power loss within such transmissions and reduces their overall efficiency.
CVTs overcome the torque interruption and speed jump problems, and can provide continuously variable output speed while keeping the input speed constant. CVTs are usually more mechanically efficient tin stepwise transmissions. But in general, CVTs are more complex in construction and require a sophisticated controlling system. Most CVTs are not rugged enough for handling torque of large magnitudes due to practical design limitations on the size of the contact footprint of friction surfaces in such transmissions. In addition, the speed variation range is somewhat limited.
Using a direct electric drive has the benefit of providing continuously variable output speed, and can achieve wide speed variation range. Such systems are appealing from a construction point of view. However, direct electric drives tend to be more costly, as they also require a generator to convert the mechanical energy to electrical energy. Since the generator and motor are in a series link with the engine and driving wheels, large motors and generators, capable of providing maximum rated power, must be used. In direct electric drives, energy is converted at lest twice between mechanical and electrical forms. Energy is lost in these conversions. Today, direct electric drives are, at best, 80% efficient.
Therefore, a need exists for a transmission capable of offering smooth and wide speed variation and yet having a high overall efficiency.
The present invention resides in a transmission including two planetary units organized about a common axis, with each including a sun member, a ring member surrounding like sun member, planet elements located between and contacting the sun and ring members, and a carrier member providing axes about which the planet elements rotate. Two members of the two planetary units are connected to external couplers through which power is introduced into and delivered from the transmission. The members of the two units are connected such that a planetary path extends between the external couplers, it being purely mechanical. Two of the members are connected to variators which themselves may be connected together to provide a secondary path through the transmission, and that path is characterized by the ability to vary speed and torque to the members connected to the variators.